1. Field of the Invention
The present invention relates to a synchronous control unit for synchronizing two shafts with each other in a machine tool, for example, a gear finishing machine.
2. Description of the Related Art
A machine tool, for example, a gear finishing machine includes: a tool shaft having a tool; and a workpiece shaft for holding a workpiece. The tool is rotated by a tool shaft motor around the tool shaft and the workpiece is rotated by a workpiece shaft motor around the workpiece shaft. It is necessary for the workpiece shaft and the tool shaft to be driven synchronously with each other.
In order to synchronize the two shafts with each other, the NC command synchronous system and the master-slave synchronous system are provided. FIG. 6 is a block diagram of the synchronous control unit of the NC command synchronous system. In FIG. 6, a movement command made in the control unit 100 is supplied to the tool shaft amplifier 220 through the second communication path 210 and then supplied to the tool shaft motor 230. After the movement command is multiplied by a synchronization ratio, it is supplied to the workpiece shaft amplifier 120 through the first communication path 110 and then supplied to the workpiece shaft motor 130. In other words, in the NC command synchronous system, the movement command for the tool shaft motor and the movement command for the workpiece shaft motor, which are made for synchronous operation, are respectively supplied from the control unit to the tool shaft motor and the workpiece shaft motor.
FIG. 7 is a block diagram of the synchronous control unit of the master-slave synchronous system. In FIG. 7, a movement command made in the control unit 100 is supplied to the tool shaft amplifier 220 through the second communication path 210 and then supplied to the tool shaft motor 230. Then, a position feedback value of the tool shaft motor 230 is supplied from the tool shaft amplifier 220 to the workpiece shaft amplifier 120 through the third communication path 310. In the workpiece shaft amplifier 120, the position feedback value is multiplied by a synchronization ratio and then supplied to the workpiece shaft motor 130. In this way, in the master-slave synchronous system, the movement command is supplied from the NC control unit only to the tool shaft (master shaft). A movement command, which is proportional to the position feedback value of the tool shaft, is supplied to the workpiece shaft (slave shaft). In this connection, in the case where the workpiece shaft is a master shaft and the tool shaft is a slave shaft, the foregoing substantially applies.
In a machine tool to which a relatively strong cutting disturbance of low frequency is given, the master-slave synchronous system is adopted in many cases. The reason is for this is that in the case where the frequency of disturbance is low, even when a position feedback value on the master shaft side is changed by disturbance, the slave side is moved so that it can follow the master shaft. Therefore, it is easy to maintain synchronization.
On the other hand, in the case where the frequency of cutting disturbance is high, the NC command synchronous system is adopted. The reason for this is that even when the master-slave synchronous system is adopted and a position feedback value of the master shaft is supplied to the slave shaft, the slave shaft cannot follow the master shaft. Therefore, the slave shaft is only swung by the vibrational movement command.
Accordingly, the synchronization accuracy is lowered, if anything.
In the servo-press disclosed in the official gazette of Japanese Patent No. 3853908, the NC command synchronous system is adopted at the time of high speed operation and the master-slave synchronous system is adopted at the time of low speed operation.
However, when trouble occurs in the control unit or when something abnormal occurs in the communication path between the control unit and two amplifiers, it is impossible to supply a movement command to two shafts. Therefore, it is impossible to maintain synchronization.
The present invention has been accomplished in consideration of the above circumstances. It is an object of the present invention to provide a synchronous control unit capable of synchronizing two shafts with each other even when trouble occurs in the control unit or when something abnormal occurs in the communication path between the control unit and two amplifiers.